Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
Engines may be restarted from the idle-stop condition automatically, without receiving an operator input, for example, in response to engine operating parameters falling outside a desired operating range. Alternatively, engines may be restarted from the idle-stop condition in response to a vehicle restart and/or launch request from the operator. Following an engine idle-stop, pressure may be maintained in the hydraulic line to enable transmission and driveline functionality and to reduce the engine restart time.
One example of maintaining hydraulic line pressure is illustrated by Ji in U.S. Pat. No. 7,357,213 B2. Therein, when an idle-stop condition is satisfied, an auxiliary electric oil pump is operated to supply hydraulic oil to the transmission, maintain the hydraulic pressure in the transmission at a predetermined pressure, and thereby maintain the transmission in gear. Another example of maintaining hydraulic line pressure is illustrated by Mori et al. in U.S. Pat. No. 6,736,099. Therein, the hydraulic pressure in the transmission is provided by an accumulator which is discharged at engine restart.
However, the inventors have recognized several potential issues with such systems. As one example, the system of Ji operates the electric pump uninterruptedly when the engine is stopped to maintain the transmission fluid pressure and enable a rapid vehicle launch. As such, the continuous operation of the electric pump may degrade fuel savings and accelerate pump wear-and-tear. As another example, the hydraulic circuit includes flow through components not immediately required for the engine restart, such as an oil cooling circuit. Consequently, the electric pump has to provide flow through these additional components and compensate for leakage (for example, from spool valves), thereby further degrading fuel savings.
As yet another example, following discharge, the system of Mori et al. recharges the accumulator using the electric pump, while preventing flow of transmission fluid from the pump into the remaining components of the hydraulic circuit. As such, this may substantially increase the time required to recharge the accumulator and launch the vehicle. Furthermore, this may limit the ability of the system to support successive restart events, such as multiple shut-down and restart events, as may be required during creep. In either system, the quality of the restart operation may be substantially degraded.
Thus in one example, some of the above issues may be addressed by a method of controlling a vehicle system including an engine that is selectively shut-down during engine idle-stop conditions, the system further including a hydraulic circuit comprising a hydraulically actuated transmission component, an accumulator, and an auxiliary transmission fluid pump. In one example embodiment, the method comprises, during a first idle-stop engine shut-down condition, where the accumulator pressure is above a threshold pressure, delivering pressurized transmission fluid to the hydraulic circuit from the accumulator, while disabling the auxiliary pump. The method further comprises, during a second idle-stop engine shut-down condition, where the accumulator pressure is below the threshold pressure, operating the auxiliary pump and delivering at least some pressurized transmission fluid to the hydraulic circuit from the operating pump without travelling through the accumulator.
In one example, the hydraulic circuit in a vehicle transmission may include an electrically-operated auxiliary pump, an accumulator, a hydraulically actuated transmission component, and a secondary component. The secondary component may be a component not directly involved in engine restart, such as an oil cooler. Herein, a pressure feedback-based control system may adjust and maintain pressure in the hydraulic circuit using either the auxiliary pump or the accumulator. Specifically, during an engine shut-down, operation of the auxiliary pump may be adjusted responsive to the accumulator pressure, for example as estimated by a pressure sensor. In one example, during a first idle-stop engine shut-down condition, where the accumulator pressure is below a threshold pressure, the auxiliary pump may be operated to deliver at least some pressurized transmission fluid to the hydraulic circuit without travelling through the accumulator. In another example, during a second idle-stop engine shut-down condition, where the accumulator pressure is above the threshold, operation of the auxiliary pump may be discontinued and only the accumulator may be used to deliver pressurized transmission fluid to the transmission hydraulic circuit. Accordingly, the frequency with which the auxiliary pump is operated may be substantially reduced. By reducing the operation time of the auxiliary electric pump, substantial energy and fuel savings may be achieved while also reducing component damage (to the pump and motor) due to prolonged pump use.
Additionally, during the first or second engine idle-stop engine shut-down condition, flow of transmission fluid through the secondary component (that is, a vehicle component not immediately involved in engine restart) may be stopped, for example using one or more flow control valves. These secondary components may include, for example, the oil cooler. By stopping the flow of transmission fluid to components not immediately involved in engine restart procedures, sections of the hydraulic circuit requiring flow and pressure maintenance may be reduced, thereby reducing the energy demands on the accumulator and/or electric pump. In doing so, additional fuel economy benefits may be achieved without adversely affecting engine restart times and without degrading engine restart quality.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.